Background can be seen through objects, Alpha Over doesn't work?
You just missed to check "transparent" in the render panel, under the film section.
Once you do that, your two renderlayers will have their own alpha channels and you will be able to composite them.

However, you'll notice that you no longer have the sky background. To bring it back you just need to turn the environment pass on in the planet render layer, and use it as background for your two plates.